1. Field of the Invention
The present invention relates to apparatus and methods of non-impact printing of halftone or continuous tone information and the like with small pixels (dots) that vary in grey level.
2. Description of the Prior Art
In the printing arts, various non-impact recording or printer heads for use as dot printers are known. Examples of such heads include an electrostatic printer head, an LED (light-emitting diode) array printer head, ink-jet printer head, thermal printer head, etc. While the invention will find utility with regard to non-impact printers in general, discussion will be made herein with regard to LED printer heads with which the invention is particularly suited.
In published International Application WO 91/10311, the contents of which are incorporated herein by this reference, an LED printer is described in which a series of LEDs arranged in a straight line are selectively activatable for brief periods to form a dot-like image on a recording surface. In the printer described in this patent, grey scale recording is achieved by having a digital comparator associated with each LED. At one input to the comparator, there is provided the data in the form of a plurality of digital data bits representing an exposure on-time for that LED for that PEL (picture element) recording period. At a second input to the comparator, an input from an up/down counter is provided that is rapidly changing in accordance with exposure clock signals emitted from a high speed clock. As the counter output value decreases towards zero at some point in time a match is sensed by the comparator between the two inputs of the comparator (in accordance with its operating criterion). The LED then turns on and remains on for a very brief duration until a subsequent sensing of a match by the comparator occurs during a count-up phase of the counter. In the above noted application "exposure space" is enhanced by providing a programmable clock whose periodicity changes within the counting cycle. That is, the exposure clock pulses can be said to be "non-linear". The gradations of exposure provided are more realistically related to human visual perception. The above application also notes that the data may be adjusted to provide also for exposure balancing of LED's. This is desirable where non-uniformity in illumination from LED to LED on the printer head may be expected.
A problem with the above is that certain printer systems are required to operate at high recording medium speeds. Where an 8-bit counter is used that counts 511 non-linear exposure clock counts for each line of pixels a problem arises as to providing sufficient time to finish the count before recommencing the counting process for recording the next line of pixels. In such printer systems only reasonable quality may be needed for printing information but an 8-bits per pixel bit-depth dimension may not be necessary. For example, a 6-bits per pixel bit-depth dimension may be satisfactory. On the other hand, other printer systems may be able to operate at slower recording medium speeds but require greater quality of continuous tone looking information and do require an 8-bits per pixel bit-depth dimension. As used herein, the term "bit-depth dimension" refers to the number of image data bits used in controlling duration of recording of a pixel or dot. In the example described herein with regard to a preferred embodiment, the bit-depth dimension refers to a corrected grey level image data signal and not just the grey level alone of the pixel to be recorded. Thus, for example, for a bit-depth dimension of 6-bits the number of grey levels for recording the pixel are 15 (not including no exposure) but 6-bits are used to define 15 assignable corrected exposure times from a set of 63 available times. Of course in a system where no correction is considered necessary, bit-depth dimension may equate directly to grey level image data.